Electronic fuel control systems are increasingly being used in internal combustion engines to precisely meter the amount of fuel required for varying engine requirements. Such systems control the amount of fuel delivered for combustion in response to multiple system inputs including throttle angle and the exhaust gas composition produced by combustion of air and fuel.
Electronic fuel control systems operate primarily to maintain the ratio of air and fuel (A/F) at or near stoichiometry. Electronic fuel control systems operate in a variety of modes depending on engine conditions such as starting, rapid acceleration, sudden deceleration, and idle. A primary mode of operation is closed-loop A/F control.
In closed-loop A/F operation, the oxygen in the exhaust gas is sensed by an oxygen sensor. The electronic fuel control system adjusts the amount of fuel being delivered in response to the output of the oxygen sensor. A sensor output indicating a rich air/fuel mixture (an air/fuel mixture below stoichiometry) will result in a decrease in the amount of fuel being delivered. A sensor output indicating a lean air/fuel mixture (an air/fuel mixture above stoichiometry) will result in an increase in the amount of fuel being delivered.
As the oxygen sensor ages, its output tends to deteriorate. For example, the sensor may take longer to switch from a lean indication to a rich indication, and vice-versa. If such a deterioration is not detected and compensated for, the fuel controller will deliver either too much or too little fuel to the engine, and consequently tailpipe emissions will increase. Accordingly, there is a need for a strategy by which the efficacy of an oxygen sensor may be accurately determined. There is also a need for accounting for a deterioration in the output of the oxygen sensor when determining the amount of fuel to be delivered to the engine.